Eleven metal plates, P, are required, six fixed and five movable. They are made of No. 22 gauge sheet brass and measure 4 x 6 inches. Six of them are placed in the grooves in the box in such a manner that an empty groove is left between each one. The six plates are then electrically connected together by soldering a strip of brass across their rear ends.
Fig. 141. Sliding Plate Variable Condenser.
Five slots each 1/4 inch deep and 1/4 inch apart are made with a hack saw in a piece of brass, C, 1 1/2 inches long, 3/4 inch wide and 1/2 inch thick. A small pin 1/2 inch long is set in the center of C on the opposite side from the saw cuts. A fiber handle 1/2 inch in diameter and 1 1/2 inches long is fastened to the pin.
The five movable brass plates are set in the grooves between the six fixed plates. They are allowed to project three or four inches out of the box. The yoke, C, is then soldered across the ends so that each one of the movable plates fits into its corresponding saw cut.
The capacity of the condenser is varied by sliding the movable plates back and forth between the fixed plates.
A binding post is soldered to the yoke, C, and another one to the strip which holds the fixed plates together. Connections are made to the binding posts with lamp cord or some other flexible conductor which will not interfere with the movement of the plates.
The instrument is finished by staining the woodwork and giving it a coat of varnish or shellac.
CHAPTER XVI. TELEPHONE RECEIVERS AND HEADBANDS.
A pair of high resistance telephone receivers in nice adjustment constitute one of the most sensitive electrical instruments in existence and will detect an exceedingly weak current.